This invention relates to the economical purification of water containing soluble and sparingly or partially soluble inorganic compounds using single-stage or two-stage membrane processes that integrate membrane water purification with chemical precipitation softening and complete hardness and silica removal using ion exchange resins and silica sequestering beds, respectively.
Hardness compounds such as barium, calcium, magnesium, iron, carbonate, bicarbonate, fluoride, sulfate and silica are commonly found in surface water sources such as lakes and rivers, groundwater sources such as water wells and aquifers and in aqueous industrial effluents including cooling tower blow-down, boiler blow-down and landfill leachates. These sparingly soluble contaminants limit the percentage recovery of purified water permeate from reverse osmosis (RO) and nano-filtration (NF) membrane systems, as they tend to form scale compounds upon concentration, which deposit, often irreversibly, on the surface of the membranes and reduce their useful service life.
In order to prevent premature fouling and deposition of scale compounds on the RO or NF membrane surfaces, the raw water is pre-treated by adding acid to increase the solubility of “temporary” hardness compounds, by using ion exchange softening to remove hardness ions, or by chemical precipitation of the hardness compounds and silica using “cold lime” or “hot lime” softening processes. When chemical precipitation softening pretreatment is required, this step is followed by clarification, preferably using solids contact clarifier and filtration using gravity or pressure sand filters, multimedia filters or “fine media” pressure filters. Alternatively, the clarified supernatant can be filtered by ultrafiltration or microfiltration membranes where any entrained suspended solids and fine colloids from the clarifier are completely removed, producing a membrane filtrate with very low 15-minute silt density index (SDI15) of <3 which is suitable for purification and desalting using reverse osmosis (RO) membranes or nanofiltration (NF) membranes.
In order to increase the recovery percentage of RO or NF membrane permeate, the pre-concentrated sparingly soluble compounds can be further precipitated by the addition of lime or sodium hydroxide in an inter-stage RO concentrate softening step, followed by additional clarification of the precipitated hardness compounds and silica, filtration of the clarifier supernatant and purification through a second RO or NF membrane stage to achieve further permeate recovery. However, these processes are limited to achieving overall (i.e. total) 2-stage membrane system recoveries in the range 80%-85%.
A survey of prior art shows the following patents:
U.S. Pat. No. 4,000,065 discloses the use of a combination of reverse osmosis (RO) and ultrafiltration (UF) to separate organic material from the aqueous stream. The contaminated aqueous stream is circulated from the high pressure compartment of an RO unit to the high pressure compartment of a UF unit, then to the low pressure compartment of the UF unit and then back to the high pressure compartment of the RO unit.
Japanese Patent 57-197085 discloses a filtration apparatus that comprises connecting UF apparatus and RO apparatus in series so as not to deposit scale on the RO membrane.
U.S. Pat. No. 3,799,806 discloses purification of sugar juices by repeated ultrafiltration and reverse osmosis purification steps.
U.S. Pat. No. 4,083,779 discloses a process for treatment of anthocyante extract by ultrafiltration and reverse osmosis treatments.
U.S. Pat. No. 4,775,477 discloses a process for extraction of cranberry presscake wherein the presscake is ground and subjected to microfiltration to remove colloidal high molecular weight compounds followed by reverse osmosis to recover a red-colored solution.
U.S. Pat. No. 5,182,023 discloses a process for removing arsenic from water wherein the water is first filtered to remove solids then passed through an ultrafilter, followed by a chemical treatment to adjust pH to a range from about 6 to 8. Thereafter, scale-inhibitors and anti-fouling materials are added before subjecting the water to reverse osmosis to provide a stream having less than about 50 ppb of arsenic.
Japanese Patent 53025-280 discloses the separation of inorganic and organic compounds from a liquid by first using a reverse osmosis membrane and then using a second reverse osmosis membrane having a more permeable membrane such as a microporous or ultrafiltration membrane. Part of the contaminated liquid obtained from the first membrane is processed through the second membrane.
U.S. Pat. No. 5,501,798 discloses a high recovery water purification process involving the use of reverse osmosis followed by chemical precipitation of hardness compounds from the RO concentrate followed by microfiltration to separate precipitated solids and recycling of the “suspended solids' free concentrate” back to the RO.
U.S. Pat. No. 5,925,255 and U.S. Pat. No. 6,537,456 disclose a process in which the calcium and magnesium hardness in the raw water is completely removed using hydrogen form or sodium-form weak acid cation ion exchange (IX) softening resin, followed by pH elevation by adding sodium hydroxide to increase the silica solubility and prevent its precipitation as membrane permeate recovery is increased. The pH elevation also mitigates biological fouling. This process achieves a permeate recovery of 90% or more, depending on the raw water TDS and membrane system operating pH.
U.S. Pat. No. 6,113,797 discloses a 2-stage high recovery membrane process, where the pre-concentrated hardness and silica in the RO or NF membrane concentrates are removed by chemical precipitation or by ion exchange if silica is not present in limiting concentrations, since silica is not removed by IX resins. This prior art process discloses softening and recycling of the high TDS 2nd stage membrane concentrate and blending it with the first stage RO membrane concentrate to enable further purification and water recovery from the second stage, thereby achieving overall permeate recoveries >95% in an economical manner, without resort to using costly multiple, discreet inter-stage softening and membrane stages.
U.S. Pat. No. 6,461,514 discloses a single stage high recovery membrane process, where the pre-concentrated hardness in the RO or NF membrane concentrates is removed by ion exchange. The softened high TDS membrane concentrate is recycled and blended with the raw Influent Water to enable further purification and water recovery, thereby achieving overall permeate recoveries >95% in an economical manner.
In all Prior Art silica-limiting applications where the hardness and silica compounds are precipitated simultaneously by elevating the pH with calcium hydroxide (lime), sodium hydroxide or other alkali solutions, it is necessary to provide effective and intimate contacting between the high pH water and the precipitated suspended solids. Efficient solids contacting will improve the extent of reaction of hardness precursors with the chemical reagents, resulting in higher hardness and silica precipitation efficiencies. The lower the residual sparingly soluble hardness and silica compounds in the clarifier supernatant, the higher the extent of achievable permeate recovery by the membrane system since the recovery is limited by solubility of these compounds, which will further concentrate over the membrane surface. The precipitated solids are removed by using a coagulant and/or a polymeric flocculant to enhance settling of the solids, enabling their removal from the bottom of the clarifier, while reducing the concentration of entrained fine particles that may be carried over into the filtration train downstream.
There have been many clarifier process enhancements aimed at maximizing the efficiency of precipitation and subsequent settling and removal of hardness compounds and silica. Recycling of slurry containing precipitated solids from the bottom of the clarifier to the mixing/reaction zone has been practiced for decades. Some companies have introduced a method of seeding of hardness particles to provide nucleation sites that will enhance the effectiveness of the precipitation process. Others introduce inert sand-like particles of relatively small particle size to provide a large contacting surface over which the precipitation reactions can take place, achieving higher hardness and silica precipitation rates, thus enabling use of smaller retention times in the clarifier and reducing its capital cost. However, this process is rather complex, involving separation and recovery of the inert solids in an external cyclonic separator and recycling back to the clarifier, with the loss of some of the inert solids, thus adding to the waste sludge volume generated and increasing the operating and maintenance costs.
As it can be seen, these prior art processes have limitations since they can not ensure very low residual hardness and silica concentrations in the clarifier supernatant on a consistent basis. There are many variables that affect the clarifier performance, including the influent water temperature, the pH, the dosage of alkali chemicals, coagulants and flocculants which are dependent on the flowrates and the concentrations of sparingly soluble compounds in the influent water. Furthermore, the equipment enhancements described above, while offering increased effectiveness and improved precipitation performance, are costly and involve substantial additional operating and maintenance costs. Significant concentrations of hardness cations (i.e. calcium and magnesium) still remain in the clarifier supernatant.
Since it is critical to maximize the overall membrane process permeate (i.e. purified water) recovery, in view of increasing worldwide water costs, water shortages and the escalating need for municipal and industrial water reclamation, what is needed therefore is a reliable process that is less susceptible to the above-described limitations. What is needed is a process and processes that will ensure very high efficiencies of removal of the hardness and silica compounds and achieve high overall recoveries >95%, irrespective of the influent water quality, the influent water hardness and silica concentrations, flowrate or operational problems and inefficiencies associated with the “solids” precipitation equipment.